Fresh view of light signal transduction in plants

Abstract

Through photosynthesis, light provides the energy source for plants and, ultimately, for all living organisms. In response to a fluctuating environment, the nonmotile plant must be able to sense varying light signals and to optimize growth and development. Higher plants possess sophisticated photosensory and signal transduction systems to monitor the direction, quantity, and quality of the light signal and to adjust their growth and development through regulated gene expression at every stage of their life cycle, such as germination, seedling development, and flowering. These light-regulated developmental processes are collectively termed photomorphogenesis. Seedling development of higher plants best illustrates the effect of light on plant development. Seedlingsof dicotyledonous Arabidopsis thaliana are able to follow two distinct strategies of development, skotomorphogenesis in darkness or photomorphogenesis in light (Figure 1). Darkgrown seedlings have long hypocotyls, unopened apical hooks, and undeveloped (small and unopened) cotyledons that contain etioplasts. In contrast, l ight-grown seedlings have short hypocotyls, no apical hooks, open and enlarged cotyledons with developed chloroplasts, and a distinctly different pattern of gene expression from that observed in dark-grown plants. At least three families of photoreceptors, phytochromes (red and far red light), blue light receptors, and ultraviolet (UV) light receptors, are utilized to sense the different light wavelengths, and the signals transduced by these receptors coordinately regulate the transcription of specific genes. This review will focus recent progress in the identification and characterization of key components of the light signaling process that controls seedling development. Owing to space limitations, it will concentrate on the genetic analysis of light signaling in the representative system of A. thaliana. Several excellent recent reviews deal more extensively with the photoreceptor phytochrome (Quail, 1991; Furuya, 1993; Vierstra, 1993) with light regulation of gene expression (Gilmartin et al., 1990; Thompson and White, 1991; Kaufman, 1993) and with photomorphogenic mutations (Chory, 1993). Phytochromes Phytochromes are the best-characterized plant photoreceptors, which in higher plants are encoded by a family of genes (e.g., PHYA, PHYB, PHYC, PHYD, and PHYE in A. thaliana). Also, biochemical evidence indicates that at least two types of phytochromes are present: type I, which accumulates to high level only in dark-grown plants, and type II, which is present in both darkand light-grown plants. On the basis of experiments with specific antibodies, it has recently been shown that type I and II phytochromes are encoded by the PHYA and PHYB genes, M inireview